code.gs

// Purge messages automatically after how many days?
var DELETE_AFTER_DAYS = 7

// Maximum number of message threads to process per run. 
var PAGE_SIZE = 150

/**
 * Create a trigger that executes the purge function every day.
 * Execute this function to install the script.
 * 
*/
function setPurgeTrigger() {
  ScriptApp
    .newTrigger('purge')
    .timeBased()
    .everyDays(1)
    .create()
}
/**
 * Create a trigger that executes the purgeMore function two minutes from now
 */
function setPurgeMoreTrigger(){
  ScriptApp.newTrigger('purgeMore')
  .timeBased()
  .at(new Date((new Date()).getTime() + 1000 * 60 * 2))
  .create()
}

/**
 * Deletes all triggers that call the purgeMore function.
 */
function removePurgeMoreTriggers(){
  var triggers = ScriptApp.getProjectTriggers()
  for (var i = 0; i < triggers.length; i++) {
    var trigger = triggers[i]
    if(trigger.getHandlerFunction() === 'purgeMore'){
      ScriptApp.deleteTrigger(trigger)
    }
  }
}

/**
 * Deletes all of the project's triggers
 * Execute this function to unintstall the script.
*/
function removeAllTriggers() {
  var triggers = ScriptApp.getProjectTriggers()
  for (var i = 0; i < triggers.length; i++) {
    ScriptApp.deleteTrigger(triggers[i])
  }
}

/** 
 * Wrapper for the purge function
*/
function purgeMore() {
  purge()
}


/**
 * Deletes any emails from the inbox that are more then 7 days old
 * and not starred or marked as important.
 */
function purge() {
  removePurgeMoreTriggers()
  
  var search = 'in:inbox -in:starred -in:important older_than:' + DELETE_AFTER_DAYS + 'd'    
  var threads = GmailApp.search(search, 0, PAGE_SIZE)
  
  if (threads.length === PAGE_SIZE) {
    console.log('PAGE_SIZE exceeded. Setting a trigger to call the purgeMore function in 2 minutes.')
    setPurgeMoreTrigger()
  }
  
  console.log('Processing ' + threads.length + ' threads...')
  
  var cutoff = new Date()
  cutoff.setDate(cutoff.getDate() - DELETE_AFTER_DAYS)
  
  // For each thread matching our search
  for (var i = 0; i < threads.length; i++) {
    var thread = threads[i]
    
    // Only delete if the newest message in the thread is older then DELETE_AFTER_DAYS
    if (thread.getLastMessageDate() < cutoff) {
      thread.moveToTrash();
    }
  }
}